The invention relates to an electric lamp/reflector unit comprising
a reflector body including a reflector portion having a concave reflecting surface with an optical axis and, integral therewith, a hollow neck-shaped portion about the optical axis,
and an electric lamp provided with a gastight light-transmitting lamp vessel
and comprising a quartz-glass wall which encloses a cavity of a predetermined, in general, spherical or elliptic shape with a geometric center, said wall being at least partly provided with an infrared-reflecting and visible light-transmitting coating, and said cavity accommodating a, substantially, linear electric light source,
and said lamp also comprising a metal foil which is entirely embedded in the wall and connected to the electric light source,
and comprising a first end portion and a second end portion which are arranged so as to be opposite to each other and which both comprise a seal, through which seals a respective current conductor, which is connected to the embedded metal foil, issues from the lamp vessel to the exterior.
The invention further relates to an electric lamp for use in an electric lamp/reflector unit.
Such electric lamp/reflector units are used as a source of white light for general and decorative lighting applications.
An electric lamp/reflector unit of the type mentioned in the opening paragraph is known from EP-A 0 397 422. In said document a description is given of a so-called double-ended halogen lamp provided with an infrared-reflecting interference filter, which lamp is arranged in a reflector body of the PAR 38 type, where the abbreviation PAR stands for Parabolic Aluminium Reflector, and the number xe2x80x9c38xe2x80x9d indicates a diameter of the reflector body at the location of the light emission window, the diameter being obtained by multiplying said number with an eighth of an inch, 1 inch being 25.4 mm, so that the diameter of the emission window of a PAR 38 reflector is 38xc3x97xe2x85x9 inch≈121 mm. It is an object of the inventors to enable such a double-ended halogen lamp to be fitted also in known PAR reflector bodies of a smaller physical size, for example in a PAR 20 reflector body having a diameter of the emission window of 20xc3x97xe2x85x9 inch≈63.5 mm, and, particularly, in a PAR 16 reflector body having a diameter of the emission window of 16xc3x97xe2x85x9 inch≈51 mm. An alternative embodiment of the reflector body is referred to as MR 16 (xe2x80x9cmetal reflectorxe2x80x9d), which, in comparison with a PAR reflector body having a corresponding diameter indication, has a neck-shaped portion which, viewed along the optical axis, is much shorter.
In order to bring about a satisfactory ratio between the reflector body and the double-ended electric (halogen) lamp, it is desirable to reduce the dimensions of the known electric lamp; however, this causes the temperature balance of such a miniaturized electric lamp to be adversely affected. In general, such an adversely affected temperature balance has a negative effect on the service life of the electric lamp in the electric lamp/reflector unit.
It is an object of the invention to adapt the dimensions of the electric lamp so as to make the electric lamp suitable for use in reflector bodies whose dimensions are smaller than the dimensions of the known electric lamp/reflector unit, and said adaptations also obviating the above drawbacks.
To achieve this, the electric lamp/reflector unit is characterized in accordance with the invention in that
the electric lamp is arranged in the reflector body in such a manner that the first end portion is at least partly situated in the neck-shaped portion, the cavity is situated within the reflecting portion and the electric light source is predominantly situated on the optical axis,
and in that the ratio of the distance dcI;dcII from the connection point where the current conductor is connected to the metal foil to the length lepI;lepII of the first and the second end portion lies in the range dcI/lepIIxe2x89xa70.75 and dcI;lepIIxe2x89xa70.75, the distance dcI;dcII and the length lepI;lepII being measured with respect to the geometric center of the lamp vessel.
By arranging one of the end portions of the electric lamp at least partly in the neck portion of the reflector body, it is achieved, viewed along the optical axis, to reduce the relative height of the electric lamp with respect to the reflector body, which has a favorable influence on the ratio of the dimensions of the double-ended electric lamp with respect to the dimension of the reflector body of the known electric lamp/reflector unit. In the known electric lamp/reflector unit, a double-ended halogen lamp is bodily arranged in the reflector portion of the reflector body by means of so-called mounting legs. By securing, in accordance with the invention, the first end portion of the electric lamp in the neck portion of the reflector body, a sturdy and reliable connection of the electric lamp with the reflector body is achieved. In addition, the positioning of the electric light source on the optical axis of the reflector portion is improved thereby, the electric light source preferably being positioned such that the geometric center of the electric lamp is situated in the focus of the concave reflecting surface. The improved positionability results in a higher light output and a better light distribution of the electric lamp/reflector unit.
The inventors have recognized that the miniaturization of the electric lamp, which makes it possible and attractive to fit said lamp in a reflector body having much smaller dimensions than the dimensions of the known electric lamp/reflector unit, causes, in particular, the so-called pinch temperature of the electric lamp to increase, which adversely affects, in particular, the service life of the electric lamp. The temperature of the pinch of a lamp is measured at the location of the connection point (generally formed by a welded joint) of the (external) current conductor and the metal foil embedded in the wall of the electric lamp. In general, a high pinch temperature enhances corrosion of the metal foil and/or the external current conductor. Corrosion leads to failure of the lamp as a result of the current supply being interrupted. Other causes of failure include, for example, leakage of the lamp vessel or explosion of the lamp. If in the electric lamp in accordance with the invention, the connection point between the metal foil and the external current conductor is positioned as faraway from the heat source (the electric lamp) as possible, a reduction of the pinch temperature is obtained.
Experiments have shown that a lower limit of dcI/lepI=0.75 and dcII;lepII=0.75 is desirable, in spite of the fact that the pinch temperature of the electric lamp in the electric lamp/reflector unit in accordance with the invention is higher than the pinch temperature of the known electric lamp/reflector unit, in order to achieve that the service life of the electric lamp in the electric lamp/reflector unit in accordance with the invention is at least comparable to the service life of the known electric lamp/reflector unit.
Preferably, the ratio of the distance dcI;dcII to the length lepI;lepII lies in the range 0.8xe2x89xa6dcI/lepIxe2x89xa60.95 and 0.8xe2x89xa6dcII/lepIIxe2x89xa60.95. A lower limit of dcI/lepI=0.8 and dcII;lepII=0.8 causes the service life of the electric lamp in the electric lamp/reflector unit in accordance with the invention to be longer than the service life of the known electric lamp/reflector unit. An upper limit of dcI/lepI=0.95 and dcII/lepII=0.95 has the advantage that the part of the end portion situated between the connection point and the end of the end portion gives sufficient mechanical support to the (external) current conductor during the service life of the electric lamp. As a result, the risk that the current conductor breaks off during the service life of the electric lamp/reflector unit is reduced.
An embodiment of the electric lamp/reflector unit is characterized in accordance with the invention in that the length lepI;lepII of the first and the second end portion lies in the range 12xe2x89xa6lepIxe2x89xa622 mm and 12xe2x89xa6lepIIxe2x89xa622 mm. If the first end portion is secured in the neck-shaped portion, such a length of the end portions enables the electric lamp to be fitted in a so-called PAR 20 reflector body.
An extremely favorable embodiment of the electric lamp/reflector unit in accordance with the invention is characterized in that the length lepI;lepII of the first and the second end portion is lepIxe2x89xa618 mm and lepIIxe2x89xa618 mm. Such a length of the end portions enables the electric lamp to be fitted in a so-called PAR 16 reflector body and in a so-called MR 16 reflector body.
The safety of the electric lamp/reflector unit is enhanced if the emission window of the reflector body is closed by means of a lens. In this manner, it can be precluded that inflammable objects come into contact with hot parts of the lamp. In addition, such a lens can limit the risks as a result of an explosion of the lamp vessel. The lens may be fixed to the reflector body by means of an adhesive, for example silicone paste. Alternatively, the lens can be secured mechanically, using, for example, a metal ring which is rolled over the reflector body. A clamping ring or a number of clamps may alternatively be used. The lens may be flat or curved.
In a favorable modification, the reflecting portion has a substantially cylindrical end portion near the lens. By virtue thereof, the volume within the reflecting portion can be larger so as to obtain a lower overall temperature, if so desired, without the increase in volume leading to an increase in diameter of the unit. It is alternatively possible to provide the reflector body at the outside with a profiled, for example rippled, surface. As a result, the surface area is increased, enabling a greater heat emission.
An alternative embodiment of the electric lamp/reflector unit is characterized in accordance with the invention in that the length lepI of the first end portion in the neck-shaped portion of the reflector body is substantially smaller than the length lepII of the second end portion. An advantage of providing the first end portion of the electric lamp in the neck portion of the reflector body resides in that, via the connection to the neck-shaped portion, using, for example, a heat-conducting and/or ceramic glued joint, the relevant end portion has a good heat dissipation by means of heat conduction, which reduces the pinch temperature of the first end portion with respect to the second end portion. At a constant pinch temperature of the first and the second end portion, it is sufficient to use a first end portion which is shorter than the second end portion.
Preferably, the ratio of the length lepI of the first end portion to the length lepII of the second end portion lies in the range 0.7xe2x89xa6lepI/lepIIxe2x89xa60.9. Dependent upon the connection used between the first end portion and the neck-shaped portion of the reflector body, a reduction in length of the first end portion by 10-30% results in a comparable pinch temperature for the first and the second end portion.
The electric lamp may be an incandescent body, for example in a halogen-containing inert gas, or an electrode-pair in an ionizable gas. An alternative embodiment of the electric lamp/reflector unit is characterized in accordance with the invention in that the electric light source comprises an incandescent body having a length lib, and the ratio of the length of the incandescent body lib to the length lca of the cavity in the wall, measured along the optical axis, lies in the range 0.25xe2x89xa6lib/lcaxe2x89xa60.35. Particularly if the cavity of the lamp vessel is in the shape of an ellipse provided with an infrared-reflecting and visible light-transmitting coating, and if a spiral-shaped linear incandescent body is arranged in the cavity, then said length ratio enables a very favorable reflection and transmission characteristic of the electric lamp to be obtained. The heat in the form of infrared light generated by the incandescent body is thus very effectively reflected back to the incandescent body, as a result of which the end portions become less hot, so that the pinch temperature of the electric lamp is lower.
The above-mentioned ratio of the length of the incandescent body to the length of the cavity in the wall, measured along the optical axis, can also be considered an invention in itself.
A particularly favorable embodiment of the electric lamp/reflector unit in accordance with the invention is characterized in that an internal current conductor which connects the electric light source with the metal foil is bent such that the electric light source is substantially situated on the optical axis. The better the electric light source is centered with respect to the spherical or elliptic shape of the cavity, the more effective the action of the infrared-reflecting coating and the higher the efficiency of the electric lamp is. In addition, the better the electric light source is centered with respect to the geometric center, which is preferably situated on the optical axis of the reflector body, the better the light distribution is as a result of the reflection from the concave reflecting surface of the reflector body.
The above-mentioned bending of the internal current conductor can also be considered an invention in itself.
In order to further reduce the temperature of the end portions, said end portions are sandblasted during the manufacture of the electric lamp. This has the advantage that the end portions are not covered with an infrared-reflecting coating, which leads to a reduction of the temperature of the end portions and hence of the pinch temperature. An additional advantage of sandblasting resides in that the surface of the end portions is roughened, so that a larger heat-radiating surface is obtained and hence overall internal light reflection in the end portions as a result of the coating is reduced.
In a preferred embodiment of the electric lamp/reflector unit in accordance with the invention, the metal foil and the first and the second current conductor are at least partly provided with a protective coating at the location of the connection point. This protective coating reduces the risk of corrosion of the metal foil and the current conductor at the location of the connection point. As a result of said corrosion protection, an acceptable service life of the electric lamp in the electric lamp/reflector unit is achieved while the risk of explosion of the lamp is negligible. Preferably, the protective coating contains chromium. It has been found that chromium can be effectively used as a protective coating on electric conductors of molybdenum and tungsten in quartz glass, and forms low-melting products with these materials. A chromium protective layer having a layer thickness in the range from 0.5 to 2 xcexcm is particularly favorable. The layer thickness of the coating is a parameter which, among other things, determines the degree of corrosion protection. If the layer thickness is below 0.5 xcexcm, then the protective layer is too thin and the corrosion protection insufficient. At a layer thickness above 2 xcexcm, too much material is used.
In a particularly favorable embodiment, the current conductor is led from the second end portion via an electroconductive connection along the reflecting surface to the neck-shaped portion of the reflector body. By arranging said connection at a short distance from the reflecting surface, instead of at a short distance from the electric light source, shadow formation, owing to this connection, by the reflecting surface is substantially reduced. An additional advantage resides in that such a position of the connection has a favorable effect on the heat balance of the electric lamp/reflector unit. Preferably, the connection is at least partly made from nickel. Nickel is a stable material having a good coefficient of heat conduction and it can also be used as a contact member of the electric lamp/reflector unit. A material which can be alternatively used for the manufacture of said connection is stainless steel.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.